1. Field of the Invention
The present invention relates to an exhaust gas filtering system having a particulate filter, which captures particulates contained in exhaust gas of an internal combustion engine.
2. Description of Related Art
Because of environmental concerns, it has been demanded to reduce the amount of particulates, which includes soot particulates and non-combusted particulates, discharged from a diesel engine. To meet this demand, an exhaust gas filtering system having a diesel particulate filter (DPF), which collects the particulates, is inserted in an exhaust pipe of the engine. In general, the DPF is made of a porous ceramic body, which defines a plurality of exhaust gas passages therein. When exhaust gas passes through porous walls of the DPF, which define the exhaust gas passages, the particulates are adsorbed and collected by the porous walls of the DPF.
When the collected particulates are accumulated in the DPF, pressure loss is increased, and the engine performance is deteriorated. Thus, the collected particulates need to be combusted and removed from the DPF to regenerate the DPF at appropriate timing. The regeneration of the DPF is performed by increasing the temperature of the DPF through a heating means, such as a burner or a heater or through supply of hot exhaust gas to the DPF in post fuel injection.
When the amount of the collected particulates of the DPF becomes equal to or greater than a predetermined amount, the collected particulates could be spontaneously combusted due to the increase in the DPF temperature. At this time, depending on the operating state of the engine, the collected particulates can be rapidly combusted to rapidly generate heat. In such a case, the temperature of the DPF is excessively increased to damage the DPF. In a case where a catalyst is supported by the DPF, some problems, such as degradation of the catalyst, could occur. This poses a significant problem on the system, which uses the DPF. The rapid heat generation tends to occur when the engine is in a decelerating operating mode under the high temperature condition of the DPF. When the engine is in the decelerating operating mode, an oxygen concentration of the exhaust gas becomes relatively high, and a degree of opening of an intake throttle valve is reduced to maintain the temperature of the catalyst. Thus, the flow rate of exhaust gas supplied to the DPF becomes relatively low, and thus the temperature of the DPF is increased.